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129
FXUS63 KILX 280847
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
347 AM CDT Fri Jun 28 2024

.KEY MESSAGES...

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- Widespread rain returns to the area today and tonight, though
  the risk for severe weather and flash flooding has decreased.

- Drier, cooler and less humid conditions evolve Saturday night
  through Monday; ideal for outdoor activities.

- Seasonably hot and humid weather returns by the middle of next
  week, extending through the holiday weekend, with near daily
  chances for showers and storms.
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&&

.DISCUSSION...
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Issued at 141 AM CDT Fri Jun 28 2024

A quick peek at radar this morning reveals an area of showers
pushing into far western Illinois. This activity is high-based and
struggling to reach the ground, with quite a bit of dry air to
overcome in the lowest 10 kft (see 00z ILX sounding).

The better opportunity for measurable rain arrives this afternoon
as a compact mid-level shortwave lifts across the Mid-
Mississippi Valley and provides life-support to a decaying
MCS/MCV. By that time, thunderstorm potential appears low with the
decaying MCS/MCV outpacing the axis of instability. Leaning on
the latest hi-res guidance, this first round of measurable
rainfall looks to stay north of I-72 -- perhaps well north.

The best opportunity for measurable rain then arrives late
tonight as strong shortwave energy pivots across the Upper
Mississippi Valley and helps drive a surface cold front into
portions of the Upper Midwest and Central Plains. A 30-40 kt LLJ
will veer ahead of the front and provide the focus for nocturnal
convection to blossom overnight.

Most of the 00z suite of CAMs suggest convection initializing over
central Missouri overnight as the LLJ moves atop some sort of
west-east surface boundary. As the cold pool matures with eastward
extent, the MCS will stake a greater claim of the LLJ --
effectively robbing central Illinois of its share of rich theta-e
air needed for strong convection. The net effect, if this
narrative comes to fruition, will be a diminished threat for
severe weather and flash flooding for central Illinois, as
storms will be more sparse with higher inflow bases.

One final note about the setup for today/tonight is that we appear
to have a column moisture and instability problem. And, it`s
quite possible that the culprit is the cut-off low positioned
along the Gulf coast. Without a good fetch of west-Gulf air surging
northward into Illinois, we`re left with recycled moisture and
what`s looking more-and-more like a disjointed LLJ. This most
likely adds up to a very non-uniform QPF field across central and
southeast Illinois through Saturday afternoon. For what it`s
worth, the 10th-90th percentile 24-hr QPF from the HREF offers
something like 0.05" - 1.25" as a reasonable envelope of expected
rainfall -- most of which is expected north of I-70.

We will need to keep an eye on model trends for Saturday with the
cold front pushing across the forecast area. The current thinking
is that convective debris from Friday night will overmatch
boundary layer recovery Saturday afternoon -- every 00z CAM
omitted convective initiation over our forecast area.

After a dry, secondary cold front makes its passage Saturday,
temperatures will cool down in a big way. Afternoon highs on
Sunday and Monday are forecast to be in the upper 70s, with
overnight lows cooling into the 50s Sunday night, per
deterministic NBM guidance. This modified Canadian air mass will
not stick around long, though.

By the middle of next week, typical summertime heat and humidity
will return across the region. Mid-range global guidance is in
fair agreement that another cold front sinks south into central
Illinois sometime Tuesday night or Wednesday, with it stalling-out
over or just south of the forecast area. This could put us in a
position for daily thunderstorm chances Tuesday-Friday depending
mostly on where the 594mb heat dome sets up. The NBM continues to
hedge this time period with daily 40% PoPs.

MJA
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&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1047 PM CDT Thu Jun 27 2024

High pressure over the eastern Great Lakes will gradually shift
eastward tonight and Friday, allowing increasing mid and upper
level cloud cover to move in overnight, and showers to gradually
shift into the area as well, likely only from around KSPI-KBMI
northwestward until around 22Z. Thunderstorm chances appear low
enough that no mention has been made in TAFs until around 03Z.
Winds E 8-10 kts overnight, becoming S-SE around 10 kts by 15Z.

37

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$